populate combobox

[arkiboys]

I am currently populating a combobox as follows:

private void PopulateHealthLevels()
{
List<clsStudentDetails> healthlevels = new List<clsStudentDetails>();
healthlevels = clsStudentAccess.GetHealthLevels();
foreach (clsStudentDetails details in healthlevels)
{
cboHealth.Items.Add(details.HealthLevelName);
}
}

Question:
This is what I have in xaml:
Name="cboHealth" SelectedIndex="{Binding Path=HealthLevelID}"

The selectedindex does not seem to be working correctly.
No matter which student I choose, the item shown is always the same.
Note that there are items populated in th ecombobox.
Thanks